Study On The Mechanism Of Collaborative Regulation Of Earthworms And Microorganisms In Nitrification In Cd Contaminated Soil

Nitrification,as one of the important links in soil nitrogen cycle,is sensitive to cadmium?Cd?pollution and directly affects the nitrogen supply of plants in polluted soil.Well-known as“ecosystem engineer”,earthworms could effectively improve soil physical,chemical,and biological properties,and influence various soil biogeochemical processes,including the nitrogen cycle.In this study,the endogeic species of Amynthas robustus Perrier?A.robustus?was chosen as a test organism to explore the role of earthworms in the regulation of nitrification in C d-contaminated soils.Through the earthworm tolerance and microcosmic incubation experiments under different Cd contamination gradients,the effects of earthworms on soil nitrification under different Cd contamination levels and related microbiological mechanisms were studied,the in-situ ammonia-oxidizing microorganisms before and after earthworm inoculation in Cd-contaminated soil were identified using DNA stable isotope probe?DNA-SIP?combined high-throughput sequencing?HTS?technology,and finally the role and mechanism of earthworms in improving ecosystem function of soil nitrogen cycling was explored.This study could provide a theoretical basis for solving the problem of nitrogen cycle functional degradation and nitrogen supply in the process of phytoremediation of heavy metals-contaminated soils.The main results and conclusions are as follows:?1?A.robustus has certain tolerance to Cd stress,with a tolerant concentration of 6mg·kg^-1.In both contaminated and non-contaminated soil,A.robustus significantly increased the pH of the acidic lateritic red soil,decreased the pH of the neutral sandy clay soil,slowed down the consumption of dissolved organic carbon?DOC?during the cultivation process,and significantly increased soil dissolved nitrogen content.?2?Along with the incubation time,earthworm significantly increased ammonium nitrogen in the both tested soils and nitrate nitrogen in the neutral sandy clay soil,and had no significant effect on the nitrate nitrogen in the lateritic red soil,indicating that earthworm could promote ammonification of both soils,promote potential nitrification of the lateritic red soil and both net nitrification and potential nitrification in the neutral sandy clay soil.?3?In each treatment of the acidic soil,AOA abundance was 1 to 2 orders of magnitude higher than AOB abundance.By the 42th day,in the control soil without Cd pollution,earthworm treatment significantly increased the abundances of soil bacteria,archaea,AOA and AOB.In neutral sandy clay soil,AOB abundance was higher than AOA abundance by about one order of magnitude.Earthworm increased the bacterial abundance but had no significant effect on the archaeal abundance in the neutral sandy clay soil throughout the incubation period.After an incubation of 30 days,earthworm increased tAOB abundance and significantly reduced the AOA abundance in the neutral soil.?4?Under the earthworm inoculation density and Cd pollution in this study,earthworm and Cd both showed no significant influence on the soil bacterial diversity,richness and evenness.Proteobacteria,actinobacteria,etc.were the main Phylum of bacteria,Sphingomonas,Bacillus,etc.were the dominant genus in each treatment in the neutral soil.The results of PCA show that after the incubation,the soil bacterial community composition in contaminated soil and non-contaminated soil were more different,while the differences in the bacteria community composition between earthworm and no-earthworm treatments at the same Cd levels were much less.?5?Based on whether the bacterial and archaeal amoA gene abundances in DNA-SIP samples showed obvious¹²C and¹³C peak shifts or not with buoyant densities,it was found that AOB was marked with ¹³C in both earthworm and no-earthworm treatments,whereas AOA was only labeled¹³C in no-earthworm treatment.In earthworm treatments,AOB rather than AOA plays a dominant role in soil nitrification.Using DNA-SIP combined with HTS technology,it was further found that earthworms promoted soil nitrification mainly by increasing the relative abundance of Nitrospira and Nitrosospira.In summary,earthworm could significantly promote nitrification in Cd-contaminated soil,increase the abundances of bacteria and AOB,and have little effect on the AOA abundance in the soil.Earthworms influence the nitrification process of Cd-contaminated soils mainly by affecting the soil bacterial community composition,especially the abundance of AOB.